Removed imports of dbm, it's never used

I am using conda (Anaconda from Continuum Analytics) to manage my python virtual environments. The python program seems to not have the dbm module compiled into it so was getting import errors on that module when using ant-downloader.

After searching the code, I only found import statements for the dbm module, I never saw it used. I therefore removed those imports and installed my fork with pip install git+git://github.com/binnisb/python-ant-downloader.git and it ran, fetched my data of my 410 and uploaded to garmin.

So if you don't know any reason why this module needs to be installed, it would be great if you could merge this PR.